Based on the context and given that you are writing for an English-speaking audience traveling in Japan, I'd recommend simply romanizing the Japanese so that travelers can better communicate with locals.

Other travel writers have romanized this concept as "drop-by" or "day-use" onsen/baths/hot springs if you decide to go that route.

http://outdoorjapan.com/magazine/column_details/201
日帰り温泉 (higaeri onsen) / 立ち寄り温泉 (tachiyori onsen) day-use hot spring

http://www.visitjapan2010.com/sg/articles/1011_01.html
At many ryokans throughout Japan, including those at the Hakone Onsen (Kanagawa) and Beppu Onsen (Oita), you can try baths called "tachiyori-yu" (drop-by bath) without an overnight stay. Some public spaces may also offer you these drop-by baths.